This invention relates generally to fuses, and more particularly to fuses having fuse state indicators.
Fuses are widely used overcurrent protection devices having a primary fusible link or primary fuse element assembly extending between conductive terminal elements, such as end caps or blade terminals. When the terminal elements are electrically connected to an energized electrical circuit, current flows through the primary fusible link or fuse element assembly. The primary fusible link or fuse element assembly is designed to physically melt, disintegrate, or otherwise structurally fail when the current flowing through the fuse reaches a predetermined level, thereby opening the electrical circuit through the fuse and protecting associated electrical equipment and components from damage. Once the fusible link has operated and opened the circuit, the fuse may be removed and replaced with another fuse to once again complete the circuit.
Identifying open fuses for replacement is not always an easy endeavor, especially in larger electrical systems employing a large number of fuses. To assist in locating opened fuses in a more efficient manner, various types of fuse state indicators have been developed which may be used to indicate an operative state of the fuse. Known indicators, for example, provide local fuse state identification features built into a housing of the fuse, such as lamps, light emitting diodes, temperature responsive materials, mechanical pins and flags which indicate opened fuses via a change in appearance of the fuse. Such indicators, however, are disadvantaged in several aspects.
For example, mechanical indicators have been developed including spring loaded elements which are released when the primary fuse element opens. Such mechanical indicators, however, can be difficult to assemble and may not be as reliable as desired.
Other types of fuse state indicators indicate an operative state of the fuse by a physical transformation in the appearance of a fuse. Typically, this entails a conductive indicator link assembly extending on an exterior surface of the body of the fuse. The external link forms a secondary fuse link external to the body of the fuse and is connected in parallel with the primary fuse element within the body of the fuse. Either the indicator link itself or other portions of the indicator assembly changes color when the indicator link opens in an overcurrent condition. These types of indicators, however, have proven difficult to reliably terminate the external indicator links to the terminal elements of the fuse.
Further, some known fuses include alarm terminals which may signal an external device (e.g., a computer or a relay) of an opened fuse. Such remote fuse state indication, however, entails a hard wired point-to-point connection between the fuse alarm terminal and the external device. Many fused systems are not wired for this purpose.
Regardless of the type of indicator used, manufacturing limitations tend to produce some variability in result or effect when fuses open, and such variability may make it difficult to locate opened fuses despite the indicators. For example, one fuse may exhibit an obvious change in appearance when the primary fuse element opens while another fuse may exhibit only a slight change when the primary fuse element is opened. The slight change may not be evident to personnel expecting to see the obvious change.
Additionally, a considerable period of time may elapse before a given fuse opens, and the lapse of time may decrease the likelihood that the indicator will operate properly. This is particularly the case for mechanically actuated indicating systems, fuse state indicators including lamps and illuminators, and fuse state indicators including temperature responsive materials which may degrade over time.
Still further, fuses having known indicators must typically be installed in a particular orientation in an electrical system so that the indicator is visible for inspection. If the fuse has to be removed from an electrical system to inspect the indicator, the value of the indicator is lost. Also, if a fuse is improperly installed, which may not be evident from a visual inspection of the fuse, current may not flow properly through the fuse and the indicator will not function as it was designed. In such circumstances, known indicators are ineffective to identify a problem fuse.